One that I have founf very easy to use is Win VNC.
http://www.uk.research.att.com/vnc/
The nice things about it are
1) It's free
2) It is completely cross platform. You can control your windows box from a Unix box, or control Unix from windows.
The downside of it is that it is sometimes a bit slow on refreshing displays when you are over the internet. But I use it to do exactly what you are trying to do- help my father muddle his way through using a computer.
Cheers.
Damon